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How Weeds Go Wild

It may not make great action footage for nature documentaries, but plants are in constant battle with each other--for space, light, water--and with soil pathogens that threaten to kill or stunt them. Now it's becoming clear just how important this subterranean struggle can be. Plants that escape their natural soil-borne enemies, and strike up alliances with friendly microbes, can become aggressive invaders.

The idea was first proposed 2 years ago by John Klironomos of the University of Guelph in Canada based on greenhouse experiments that suggested invasive weeds thrive when they're transplanted into soil lacking the pathogens that plague them in their native environment (ScienceNOW, 2 May 2002).

New research by Ragan Callaway of the University of Montana, Missoula, and colleagues strengthens that conclusion. The group studied spotted knapweed (Centaurea maculosa), a European plant that is now a pervasive weed in the western United States. First, they planted seeds in sterilized and nonsterilized soil from both continents. Plants in pathogen-free soil from North America grew up to 148% more than those in the nonsterilized U.S. soil. But the European plants improved much more--up to 900% when they faced no pathogens compared to when they grew in untreated soil from Europe. That suggests knapweed in Europe is held back by soil pathogens even more than is the invasive knapweed in the U.S.

Enemies clearly matter, and that's especially true in the old country. When the researchers grew knapweed in French soil, it fared better in soil that had been previously planted with bunchgrass than with knapweed--presumably because the bunchgrass soil had not accumulated knapweed-specific pathogens. But it appears that enemies aren't the whole story. Montana soil showed the opposite pattern: Knapweed planted in soil that had grown knapweed did better there than in once-grassy soil, the team reports in the 19 February issue of Nature. They think that invasive knapweed has not only escaped its natural pathogens in Montana but is modifying the soil to its own advantage, perhaps by cultivating helpful mycorrhizal fungi.

"This suggests that the contribution of soil organisms in invasiveness is two-fold: [Invasives] escape from the bad guys and [get] help from the good guys," notes Wim Van der Putten of the Centre for Terrestrial Ecology in Heteren, The Netherlands. He and others recently published similar findings on black cherry plants. Figuring out exactly which pathogens are responsible will be an important next step, adds Klironomos.